In theory, continuous mining machines can continue to operate over long stretches without a break. The length of heading which can be cut in one continuous operation is, in theory, determined by the problems of getting ventilation to the face. There is however a second problem. It is unsafe for the driver of the machine to venture into an area of the heading where the roof is unsupported. In many coal mines, permanent roof support is installed by roofing bolting. Installation of roof bolts takes a certain amount of time and, more importantly, can only be carried out where access to the roof can be obtained perpendicularly or substantially perpendicularly below the spot where the bolt is to be installed.
One technique which is currently used is to first install permanent roof support right up to the face, then for the continuous mining machine to mine the face until the driver of the machine is just about to proceed beyond the supported roof. The machine is then withdrawn and cuts in an adjacent heading whilst the newly exposed roof is permanently supported using roof bolts. This is an extremely slow operation and fails to make use of the potentialities of a continuous mining machine.
It has been proposed to use a full heading shield to overcome this problem. The full heading shield is a structure which straddles the continuous mining machine and which can "walk" along the heading as the machine advances. The shield provides temporary roof support over the machine, and roof bolting can be carried on continuously in the area uncovered by the temporary roof support behind the mining machine. A disadvantage with this technique is that the heading shield needs a heading width greater than the width of the mining machine in which to operate. The mining machine therefore has to move forwards to cut one advance, then back and sideways and forwards again to cut another advance to widen the heading before the heading shield can advance. The mining machine therefore spends a substantial time in moving to and fro from one face to another, rather than continuously mining.